1. Field of the Invention
This invention relates generally to a method and apparatus for bonding a bare semiconductor chip or die to a leadframe and, more specifically, to a method and apparatus for injecting and applying an atomized adhesive to a semiconductor device component.
2. State of the Art
A leadframe is basically the backbone of a typical molded plastic package. Leadframes serve first as a die support fixture during the assembly process and are subsequently electrically connected to the die bond pads after die-attach, as by wire bonding. After transfer molding, the leadframe becomes an integral part of the package. Generally, leadframes are fabricated from a strip of sheet metal by stamping or chemical milling (etching) and are made from various materials selected for cost, ease of fabrication, and various functional (mechanical and electrical) requirements. Typical leadframe materials generally include nickel-iron, clad strip, or copper-based alloys.
An important feature of a leadframe is its ability to channel heat from the chip to the exterior of the package which is dependent on the thermal conductivity of the leadframe material. Copper alloy leadframes are desirable from this standpoint. However, copper alloy leadframes also have high thermal-expansion rates (based on coefficients of thermal expansion) in comparison to silicon, but nearly match the expansion rates of low-stress molding compounds. Consequently, the chip-bonding material, that is, the die-attach material used to bond the chip to the leadframe, must be carefully selected. For example, silicon/gold eutectic bonding cannot be used with copper frames because its high elastic modulus couples thermally induced bending stresses generated by leadframe expansion to the silicon of the die, significantly increasing the potential for die fracture As a result, silver-filled epoxies and polyimide die-attach adhesives have been developed that are flexible enough to accommodate the stress initiated by an expanding copper leadframe so that the die is not subjected to strain.
Leadframes are typically supplied in multi-frame strips designed with automated assembly, wire-bond and packaging system in mind. As such, tooling or indexing holes are located along the leadframe-strip edges to mate with transfer-mechanism elements and alignment pins. Such pins are typically part of the assembly equipment, including die bonders, wire bonders, molds, auto-inspection stations, trim and form equipment, and marking machines.
As noted above, both silicon/gold eutectic as well as adhesive bonding materials have been used to bond the die to the leadframe. For silicon/gold eutectic bonding, the operation typically begins by indexing die on a bonding machine to a heated die support platform. Leadframes are then fed from magazines, along a track to a heater block. A small square of silicon/gold alloy (typically 6% Si, 94% Au) is cut from a feed ribbon and transferred to the die support platform, also called a die-attach paddle, tab or island. Die and eutectic are then scrubbed together, forming a hard alloy bond. The heater temperature is approximately 420.degree. C. and the total cycle time for eutectic bonding is about 6 to 8 seconds.
Adhesive bonding is faster than eutectic, with a cycle time of about 2 seconds. Typical feed mechanisms for polymer bonding machines are the same as eutectic bonders. The leadframes, however, are usually not heated. Silver or gold-filled epoxy or polyimide adhesive paste is transferred to the die support platform by a print head, and a die is pressed into the paste immediately after printing. Die bonding adhesives manufactured by Epotek, Amicon and Ablestick are typically electrically conductive, have maximum cure temperatures up to 275.degree. C., and have lap shear strengths up to 2.11 kg/mm.sup.2.
The die bonded leadframe strips are subsequently loaded into transport magazines. Eutectic-bonded frames go directly to a wire bonding station, while magazines containing adhesive-bonded frames are routed to ovens for curing. The curing atmosphere is typically dry nitrogen and usually requires one hour at 150.degree. C. to cure, followed by 30 minutes at 275.degree. C. for polyimide adhesives.
A large number of polymers, copolymers, and polymer blends have been developed in the past several decades with the aim of joining composites. Epoxies, however, have been the primary material used to bond laminates, and such adhesives have been the principal bonding agent for printed circuits. Due to their ability to react with many types of compounds and to enter solid solution with a variety of modifiers, epoxies can be formulated to meet most requirements that do not exceed their use temperature of 125.degree. to 150.degree. C. For thermoplastic adhesives requiring the adhesive material to be heated to temperatures around 200-300.degree. C., up to three hours of cure time may be required to remove any solvents. Epoxies, moreover, are generally very viscous and consequently somewhat difficult to handle and apply.
Acrylics have been customarily used as adhesives for polyimides, requiring temperatures higher than the maximum use temperatures of epoxies. Polyimidesiloxane hybrids have also been used that have superior thermal resistance and good compatibility for this purpose, For applications requiring the highest temperature or the most demanding dielectric requirements, polyperfluorocarbons may be used. Perfluorethylenepropylene copolymer films also exhibit suitable adhesion if the adherend surface is prepared properly.
Typically, rough or absorbent adherends readily bond together. Smooth, impervious materials, however, are much more difficult to adhere, and these are more typical of printed-circuit substrates. Smooth surfaces, even if clean, usually cannot be bonded unless roughening or chemical treatment of the adherend surface precedes adhesive application. Metal surfaces too smooth to be bonded can be roughened by abrasion, but more frequently unalleviated metal surfaces as on power or ground planes are prepared for bonding by chemical modification. Alkaline oxidation of copper provides an instant oxide surface more polar and irregular than the original. Chelates such as benzotriazoles bond well to the oxide that is always formed on copper and can be stable to 200.degree. C. These chelates allow good adhesion to organics, have high cohesive strength, and serve as corrosion inhibitors when used as coatings.
Typical prior art devices apply (lie bonding adhesives by rolling, stamping, tape application, or spraying. That is, an adhesive-bearing roller or a stamp may be used to apply the adhesive to the die-attach area of the leadframe. Similarly, a mask may be placed over the portion of the leadframe where adhesive is not desired and a spray nozzle may spray the masked leadframes with the desired adhesive. Other methods have also been developed, such as that disclosed in U.S. Pat. No. 5,286,679, in which a patterned adhesive layer is deposited by hot or cold screen printing the adhesive, by photopatterning a photosensitive adhesive, or by utilizing a resist method of etch back. Adhesive-coated tapes have also been utilized to bond the die to the leadframe, as disclosed in U.S. Pat. No. 5,304,842, as well as adhesive patches applied from tape carriers, as disclosed in U.S. Pat. No. 5,448,450.
The prior art systems, however, have several notable disadvantages. For example, such systems may not evenly distribute the adhesive across a die-attach paddle of the leadframe. Moreover, application of an adhesive having large particle sizes may cause voids or bubbles to form on the leadframe. Other adhesive applicator apparatuses do not draw the adhesive evenly onto the die-attach paddle or the leadfingers (in the case of a leads-over-chip, or LOC, die-attach) and may have relatively slow cycle times. In addition, many prior art systems do not fully enclose or focus the adhesive application process to substantially reduce, if not substantially eliminate, contamination of equipment components and leadframe areas where adhesive is not desired. Prior art systems may also require an additional, preliminary masking step in the manufacturing process to ensure that the adhesive is only applied to desired locations. Finally, many prior art systems may require an extended curing cycle in an oven, for example, to cure the adhesive or epoxy. In some systems, after die-attach, an optical alignment system checks to ensure that the die has been properly placed, translationally and rotationally, on the leadframe. Between die-attach and wire bonding, however, dies requiring an extended epoxy cure cycle often have their orientation or placement distorted as the adhesive cures, adversely affecting the wire-bond operation and, thus, quality and reliability of the product.
It is also desirable in some circumstances to apply adhesive to a wafer surface for so-called leads-over-chip or LOC mounting of a die under the leads of a leadframe without a die-attach paddle. Currently, screen-printing is employed to selectively coat the wafer. Alternatively, a spin-on process may be employed to coat the entire wafer surface, followed by selective etching away of the adhesive in undesired locations. Adhesively-coated tapes are also employed for LOC die-attach. Such processes, as with those previously described, leave much to be desired in terms of process time, ease of use, and resulting product quality.
Thus, it would be advantageous to provide an apparatus for applying an atomized adhesive to a semiconductor device component for die bonding that encloses the adhesive application process, evenly draws the adhesive onto the die paddle and removes excess adhesive, automatically masks or shields each component to protect certain areas from being sprayed with adhesive, and has a relatively fast cycle time.